squid

Squid are diverse cephalopods which are
highly adaptable to changing environments. They have been in existence for
tens of millions of years.

Main characteristics

Squid have a distinct head, a mantle, eight arms, and two tentacles,
the only exception being bigfin squid which has ten long, thin arms of equal
length. Squid are generally no more than 60 centimeters (23.6 inches) in length but the
giant squid can reach lengths of 13 meters (42.7 feet). In 2003 the Colossal Squid
was discovered and it is thought that they may grow to 14 meters (45.9 feet), making
them the largest known invertebrate.

Anatomy

The mantle is where the main body mass of a squid is enclosed;
on each side of it is a swimming fin. The underside of the body has openings
to the mantle cavity, which contains the gills,
and to the reproductive and excretory systems. The siphon,
located at the front of the mantle cavity, is used for jet propulsion: water
is sucked into the mantle cavity and expelled from the siphon in a strong,
fast jet. By changing the direction of the siphon, the squid is able to
move is different directions with precision.

Squid have three hearts, two which feed the gills and a larger, systemic
heart pumps copper-rich hemocyanin around
the body. The systemic heart consists of three chambers: two upper auricles
and a lower ventricle.

Attached to the head of a squid are eight arms and two suckered tentacles
that do not grow back if they are severed. The mouth has a sharp beak that
is used to kill and tear prey into manageable pieces for eating; inside
the mouth is the radula.

On each side of its head a squid has an eye equipped with a hard lens that
is focused through movement rather than change of shape. Squid have excellent
eyesight, and have acute senses of touch and taste. Giant squid have the largest eyes in the world.

The squid's skin is covered with chromatophores that enable the animal to
change color to match its surroundings. The underside of a squid is usually
lighter in color than the dorsal surface to provide camouflage from predators
and their prey.

Distribution and habitat

Across the various species, squid occupy a diverse range of habitats. Some
thrive in warm tropical waters while others prefer cooler waters. Although
they are mainly found in salt water, some species can also survive in freshwater
locations.

Squid are particularly abundant in the North Atlantic, and around the Gulf
of Mexico, Hawaii, and California. In the North Pacific they have been found
from the Bering Sea to the Sea of Japan; they also occur in the South Pacific.
Both the Colossal squid and the giant squid can be found in the deep waters
near Antarctica.

Squid are usually found at depths below 1,000 meters (3,280 feet), although some
species and youngsters can be found nearer to the surface. Squid are highly
adaptable creatures. They are found in just about all of the major bodies
of water and are able to move and thrive in different areas if they lack
a food source or their natural habitat has been destroyed.

Reproduction

To attract females, males begin their display at dawn by rapidly swimming
around in circles. Females join in and as time goes by they join up and
swim around in pairs. When a female has taken an interest, mating occurs.
During mating the male uses one of his arms to place sperm inside the female's
ink sac where the fertilization of her eggs takes place.

A single female can produce thousands of eggs and she will distribute them
in hidden places in the water. A large number of these eggs will be eaten
by predators but those that remain hatch after about eight weeks. The young
squid are able to fend for themselves from hatching. Squid have a short
lifespan, usually 1–2 years. Usually they are able to mate only once
in this time, although in some species mating can occur twice.

Diet and methods of capturing prey

Squid feed mainly on fish, prawns, and other cephalopods. They tend to catch
their prey by shooting out their tentacles, grasping the prey, and drawing
it back toward the arms. Other methods include dragging their tentacles
along the ocean floor, capturing small prey in the sticky suckers, and drawing
the prey back into their arms, or dangling their long tentacles into a school
of fish, grasping one, and lunging forward to grab the prey in the arms.

Predators and defense

Squid are preyed upon by many species of mammals, fish, and birds. The most
notable of these are the orange roughy fish, wandering albatrosses, and
sperm and pygmy sperm whales.

To avoid predators, squid are able to rapidly swim away or squirt ink into
the water to cause confusion. Some species are able to change color so they
can camouflage themselves to avoid detection.

Taxonomy

Squid belong to the order Teuthida (the largest cephalopod order) within
the class Cephalopoda. The order Teuthida is divided into two major suborders,
Myopsina and Oegopsina, which contain 29 families and about 300 known species.

Other interesting facts

Many species of squid are popular in cuisines such as Chinese, Greek,
Spanish, Italian, Turkish, Portuguese, Japanese, Korean and Indian.

Some species of squid are able to glow in the dark because they have
bioluminescent organs.

The Humboldt Squid is a very aggressive species and has been known
to attack sharks in the water.

Some species of squid are known to live at depths of more than 3,960
meters (13,000 feet).

300 species of squid have been identified but it is believed that
there at least 200 more species are awaiting classification.

The Colossal Squid is the largest invertebrate in the world.

The eyeballs of the Giant Squid are the same size as a standard basketball.